1. Field of the Invention
This invention relates to the field of pilot valves which control the operation of a main valve. In particular, the invention relates to improvements in pilot valves used to relieve pressures higher than a predetermined set pressure via a main valve on a vessel such as a tank for storing hydrocarbons.
2. Description of the Prior Art
A prior patent, U.S. Pat. No. 3,477,456 issued Nov. 11, 1969, in the name of the instant inventor describes a pressure responsive pilot controlled valve of which the invention described below is an improvement. The pilot valve of the '456 patent is disposed atop the dome of a main valve which has its inlet in communication with a tank, the pressure of which is to be sensed and controlled. A large diaphragm valve in the main valve is controlled by the pressure in the dome, which in turn is controlled by the pressure in a port of the pilot valve. (In actual practice, the pilot valve of the '456 patent has also been used to control a piston-type main valve.) The pilot valve is connected to the inlet of the main valve, and reacts to a set pressure level to lower the pressure in the port of the pilot valve and in the dome of the main valve to cause the diaphragm in the main valve to open, thereby opening a main valve flow path from inlet to outlet, thereby relieving the pressure level in the tank. When main valve inlet pressure is reduced, the pilot valve senses such reduction in pressure, raises the pressure in its port, and as a result the pressure in the dome of the main valve, with the result that the diaphragm of the main valve closes again.
The pilot valve of the '456 patent includes a diaphragm case having two diaphragms in it thereby creating a lower cavity (called a boost cavity) and a cavity between the two diaphragms (called a sense cavity). A valve spindle is connected to the two diaphragms. A pilot spring is placed between the valve spindle and the case of the valve. The lower end of the valve opens and closes about a valve seat about the port of the valve which is connected to the main valve. When the valve is open, a flow path exists between an exhaust port to atmosphere, thereby applying atmosphere pressure via the valve port to the dome of the main valve.
Under normal operating conditions, tank (or system) pressure acts on the bottom of the main valve seat and on top of the main valve diaphragm via the port of the pilot valve. The main valve seat is held tightly closed by a large force equal to the tank pressure times the differential areas of the main valve diaphragm.
Tank pressure is directly applied to the boost cavity and to the sense cavity downstream of a variable orifice from the inlet of the pilot valve which is in communication with the inlet or tank pressure from the main valve. The soft pilot seat is held closed by the pilot spindle valve because the force of the spring on the spindle is greater than the upward forces acting on the upper or sense diaphragm.
Set pressure is that pressure at which the upward force on the upper pilot diaphragm (that is, the sense diaphragm) exceeds the spring force. At such pressure, the pilot spindle lifts slightly, causing a small flow of fluid across a blowdown adjustment orifice via the port to the pilot exhaust to the atmosphere. This small flow across the blowdown adjustment orifice causes a reduction in pressure downstream of this orifice and in the sense cavity. Even though this is only a small reduction in pressure, it creates a large net upward force which rapidly opens the pilot seat. That is, it "snaps" the pilot spindle full open and immediately exhausts the main valve dome to atmosphere. As a result, the main valve rapidly opens also.
When the pressure in the tank is reduced, the lifting force created by pressures in the sense cavity and the boost cavity are equalized. As a result, the spindle constricts flow across the adjustable orifice thereby reducing sense cavity pressure which in turn reduces spindle left which urges the pilot closed thereby causing the main value also to close.
One disadvantage of the pilot valve of the '456 patent exists in that it includes a variable blowdown adjustment orifice as well as a fixed orifice. Another disadvantage of the pilot valve of the '456 patent concerns an external conduit from a point between the adjustable orifice and the fixed orifice to the sense cavity between the upper and lower diaphragms in the diaphragm case. Such external conduit provides a long path from the adjustable orifice to the sense cavity.